As shown in FIG. 1, in a conventional cold cathode fluorescent planar lamp 10, gas sealed in a cavity is first exhausted out via an exhaust pipe 15, and noble gas or mercury vapor 11 is then filled. Next, a high voltage is applied across electrodes 12 and 12′ to cause electron migration between the electrodes 12 and 12′. During the process of electron migration, electrons 16 will impact gas molecules to excite the mercury vapor 11 sealed in the cavity to radiate out ultraviolet (UV) light 13. The UV light 13 will thus excite a fluorescent powder layer 17 coated on the inner wall of the cold cathode fluorescent planar lamp 10 to emit visual light 14. In addition to the complicated procedure of vacuuming, steps of coating fluorescent powder on the inner wall of the cold cathode fluorescent planar lamp 10 needs to be performed within a high temperature range from 400° C. to 800° C. to evaporate organic solvent in the fluorescent powder layer 17, hence adhering fluorescent powder to the inner wall of the cold cathode fluorescent planar lamp 10. If the organic solvent in the fluorescent powder layer 17 is not fully evaporated, a color of burned black will occur at the electrodes of the cold cathode fluorescent planar lamp 10 after a longtime illumination. Besides, if fluorescent powders of the red, green and blue colors are not uniformly coated, shift of color temperature of the visual light may occur. Therefore, how to manufacture a visual light source with uniform color temperature and high brightness but without complicated procedures is an urgent problem to be solved in the industry.